Citrus production is one of the most important agricultural economic activities in the United States and around the world. According to the Food and Agricultural Organization (FAO) of the U.N., approximately 47,170 tons of citrus fruits such as oranges, lemons, grapefruits, etc., were produced in 2011, corresponding to US$2.3 billion from sales of fresh fruits and juices around worldwide (FAOSTAT). However, the citrus industry has been experiencing a big threat from phloem-colonizing and insect-transmitted bacterial diseases including Citrus Stubborn Disease (CSD) and Huanglongbing (HLB, also known as citrus greening). Diagnosis of these diseases has been very challenging because of the low titer and uneven distribution of the pathogens in the citrus tree.
The causative bacteria Candidatus Liberibacter (for HLB) and Spiroplasma citri (for CSD) reside exclusively in the phloem of infected trees once they are introduced by the phloem-feeding insect vectors or by grafting. So far, CSD or HLB resistant varieties of citrus have not been found.
The most commonly used pathogen detection methods are PCR based, which requires the presence of bacterial cells or DNA on the tested sample for positive diagnosis. This is been problematic because the low titer and variable distribution of the pathogen within infected trees. Both S. citri and Ca. Liberibacter cells exhibit extremely uneven distributions in infected trees. Very often, the pathogen cells cannot be detected even in symptomatic branches or leaves. Moreover, nucleic acid-based assays require sample preparations, which can be complex, costly and time consuming, especially when numerous samples have to be tested for one tree. So far, the ability to process thousands of samples necessary to track an epidemic using nucleic acid-based methods remains manpower and cost challenging.
Bacteria pathogens secrete numerous proteins to the environment and some of these secreted proteins are essential virulence factors functioning in plant cells. Genes encoding these specialized pathogenesis-related protein secretion apparatus are present in the host plant, but absent from S. citri and Ca. Liberibacter. Bacterial pathogens are injected into plant tissues, i.e., the phloem, by their corresponding insect vectors at the initial stage of infection; therefore, proteins secreted from the general secretion system are readily delivered inside the host cells. This is consistent with the observation that the pathogen cells are often absent from symptomatic tissues. While not being bound by any particular theory, it is believed that the presence of secreted proteins are responsible for the symptom development. Importantly, there are no curative methods once the trees are infected. The success of disease management is largely dependent on early pathogen detection.